1. Field of the Invention
The present invention relates to ink-jet recording method that comprises adhering flying ink (recording liquid) droplets onto recording media, thereby forming color images. More particularly, the invention relates to an ink-jet recording method for recording full-color images with a wide range of reproducibility of colors and with good gradation.
2. Description of the Prior Art
Ink-jet recording method comprises (i) producing ink droplets by a technique for discharging inks, such as the electrostatic attraction technique based on the application of high voltage, the technique of causing mechanical vibration or displacement of the ink by means of piezoelectric elements, or the technique of heating the ink to foam instantaneously and utilizing the foaming pressure, and (ii) causing the ink droplets to adhere to recording media such as paper. This recording method is noticed in that it generates limited noises and permits high speed recording.
When color images delicate in shade or tinct like those in silver salt photographs are formed by ink-jet recording, the density gradations of various colors in the images are expressed in the conventional ink-jet recording by using inks of colors, e.g. cyan, magenta, yellow and black and by controlling the diameter and number (per unit area) of the ink dots to be formed on recording media for color mixing according to the densities of three primary color components in the original images to be recorded.
However, according to this method of recording with varying the diameter and number of ink dots, it is impossible to express such gradations that image density gradually varies over the entire area of from the lowest to the highest density according to original images, resulting in low resolution images or unnatural images.
Various ink-jet recording techniques have been proposed to solve these problems. Among these, techniques of recording by use of at least two inks of different dye concentrations for each color are known from Japanese Laid-Open Patent Application No. 156264/82 and other literature.
In particular, the ink-jet recording technique disclosed in the above patent is characterized in that a plurality of inks of different dye concentrations are used for each of colors, the diameter and number of the ink dots to be formed are varied according to the density levels in the original image, and at the same time dots of one of the inks of lower dye concentration are printed over almost the entire range of from low to high density of the image to be recorded. According to this technique, the density gradations of each color can be better expressed, particularly on lower image density areas, than according to the conventional technique.
In such recording techniques, however, ink properties such as viscosity, surface tension, and pH are considerably varied from ink to ink with the difference of dye concentration among plural inks, since the inks of the same color and of different dye concentrations are prepared by dissolving a dye in different concentrations in the same composition of solvents. In particular, the viscosity variation among the plural inks used causes significant variation in the intended diameter of the resulting ink dots and large deviation between the point where each ink droplet hits and the target point on recording media. This is a cause of great difficulty in recording high quality images with high resolution and natural feeling.